1. Field of the Invention
The present invention relates to a turbine generating apparatus having a permanent magnet type synchronous generator driven by a gas turbine to generate electric power.
2. Description of the Related Art
A turbine generator with a gas turbine such as a micro-gas turbine directly connected to a synchronous generator uses a generation frequency which is the product of the rotational speed of the turbine (hereinafter, may be referred to as the number of revolutions) and the number of pole pairs of the generator and generates electric power at a higher frequency than the commercial frequency (50 or 60 Hz). For example, a generator of 2 poles (the number of pole pairs is 1) directly connected to a turbine of 60000 rpm generates 1-kHz power. Further, in such a generator of high-speed rotation, to ensure strength for withstanding centrifugal force and increase the output density as a miniature generator, a generator using a permanent magnet of a strong rare-earth element for the magnetic poles of the rotor is used. Namely, the permanent magnet type synchronous generator is, unlike a generator having a magnetic winding, a generator in which the magnetic field intensity cannot be operated and the generation voltage cannot be controlled. To utilize the generation output, it must be converted to the commercial frequency (50 or 60 Hz) and voltage (for example, 3-phase 220 V). The conventional art of a device for performing this power conversion adopts a constitution of converting the power to a direct current once by a rectifier circuit and then converting the DC power again to an alternating current at a required frequency and voltage by an inverter (for example, Japanese Patent Laid-Open Publication 11-356097).
FIG. 12 is a block diagram showing the constitution of the conventional art at the time of generation. A gas turbine 111 drives a permanent magnet type synchronous generator 112, and the output of the generator 112 is converted to a direct current by a rectifier circuit 113 realized by a diode bridge, and the direct current is converted to a voltage of the commercial AC power, for example, 220 V and a frequency, for example, 50 or 60 Hz by an inverter 114 for performing pulse width modulation (PWM).
FIG. 13 is a block diagram showing the constitution of the conventional art shown in FIG. 12 at the time of start. The power of a system 115 for supplying the commercial AC power is converted to DC power by the rectifier circuit 113, converted to a frequency suited to the generator 112, for example, 1 to 1.6 kHz by the inverter 114, and given to the generator 112. By doing this, the generator 112 operates as a motor and drives and rotates the gas turbine 111. Fuel is fed to the gas turbine 111 and ignited, and, as the output of the gas turbine 111 increases, the output of the generator is derived.
In the conventional art shown in FIGS. 12 and 13, the frequency for pulse width modulation of the inverter 114 is, as mentioned above, 50 to 60 Hz at the time of system connection shown in FIG. 12 and 1 to 1.6 kHz at the time of start shown in FIG. 13.
Further, the gas turbine generally has a property that when the output is lower than the rated output, running at a number of revolutions smaller than the rated number of revolutions improves the efficiency and when the gas turbine is to be operated in serious consideration of efficiency, the number of revolutions must be changed according to the output. On the other hand, the synchronous generator driven by the gas turbine has a property that the induced voltage is proportional to the number of revolutions and in a generator capable of controlling no magnetic field like the aforementioned permanent magnet type synchronous generator, the voltage is also changed depending on the number of revolutions. Therefore, to efficiently operate the turbine generator, regardless of frequency changes and voltage changes of the generated power, a power converter capable of converting the power to a fixed voltage and frequency is required.
The aforementioned conventional art shown in FIGS. 12 and 13 has a constitution that the generated power is converted to a direct current once by the rectifier circuit 113, and then the DC power is converted again to an alternating current at a required frequency and voltage, and the rectifier circuit 113 is formed as a passive diode rectifier circuit. Therefore, the DC voltage after rectified varies in proportion to the generation voltage. Further, when the inverter 113 of the pulse width modulation (PWM) type is used, the output voltage can be controlled to a certain extent by the PWM rate. However, the range thereof is limited and when the voltage of the DC part, to which the output of the rectifier circuit 113 is given, is changed greatly, the output voltage after AC conversion by the inverter 114 cannot be kept fixed. Namely, when the voltage of the DC part is increased, although it is possible to make the duty of each pulse of PWM (the interval that the pulse is on during one period) smaller, the adjustable range is limited. When the voltage of the DC part is reduced below a certain value, even if the duty of each pulse of PWM is maximized, the output voltage cannot be held. Further, when the voltage of the DC part inversely exceeds the limit, a voltage more than the breakdown voltage is applied to the switching power transistor of the inverter 114 and the power transistor is destroyed. Therefore, to output a fixed voltage from the inverter, the number of revolutions of the generator 112 is restricted in correspondence with the restriction to the voltage range of the DC part.
A method of enlarging the range of the number of revolutions of the generator 112 capable of obtaining a predetermined output voltage from the inverter 114 is as indicated below. Firstly, the induced voltage of the generator 111 is designed high and even if the number of revolutions is small, the voltage is ensured. Then, the breakdown voltage of the power transistor of the inverter 114 is increased so that the power transistor can withstand voltage rise during the rated rotation of the generator 112. In this case, during the rated rotation, the duty of PWM is narrowed so as to generate a predetermined voltage. Then, a problem arises that only a part of the capacity of the power transistor is used.
The aforementioned problem that only a part of the capacity of the power transistor is used is solved if a rectification method of holding the DC part at a fixed voltage is used within a wide range of the number of revolutions. With respect to it, there is a conventional art referred to as a PWM converter. The conventional art switches the transistor bridge according to the voltage of the generator by PWM and is characteristic in that it can operate the DC voltage after conversion. Therefore, as a power conversion method corresponding to changes in the generation frequency and voltage of the generator, a constitution that a PWM converter and a PWM inverter are combined via the DC part may be possible.
However, due to the restriction on the response speed of the present power transistor, the maximum PWM frequency is 10 kHz or so practically. Further, when the generation frequency is higher than 1 kHz, for example, when a microcomputer controls PWM, due to the restriction on the response speed of the microcomputer, another problem will arise that it is difficult to fit the PWM timing to the generation voltage.
An object of the present invention is to provide a turbine generating apparatus for stably converting high frequency output of a synchronous generator at a varying frequency and voltage to a required frequency and voltage and outputting the same.